Field of the Invention
The present invention relates to an image forming apparatus that can determine an image printed on a transfer medium.
Description of the Related Art
In an image forming apparatus, such as a copying machine, a printer, and a multifunction machine, printing is performed by forming toner images on photoreceptors included in an image forming unit based on image data obtained by reading the image data from a document reading apparatus, transferring the toner images to sheets fed from a tray, and fixing the toner images.
The fixing device passes the sheets to a fixing roller, a pressure belt, and the like and fixes the toner images on the sheets by heat, pressure, and the like. Sheets provided with images are conveyed to the fixing device. Therefore, in some cases, fixing members, such as the fixing roller and the pressure belt, are stained by toner over time, or the toner is attached to the fixing members by a jam of a sheet. When a sheet is newly passed to the fixing device, the toner of the fixing members is attached to the sheet, and the sheet is stained.
In some cases, the image density changes over time. Depending on the degree of the stain or the change in the image density, the abnormality of image reduces the print quality, and automatic detection of the abnormality is desired to prevent the generation of a defective product.
Japanese Patent Laid-Open No. 2012-255843 has a function of checking incorrect collating or missing pages by comparing check data (checksum) of image data (during writing) of a first copy (or during check print) and image data of second and subsequent copies, and variable areas and stamp areas that are different in each copy are not checked. In other words, the comparison is based on check data of images in which images of the areas are removed. However, this apparatus checks the write image data, and the apparatus cannot detect a stain of a printed sheet, an abnormality of image density, or the like.
Meanwhile, an image forming apparatus is proposed that reads an image of a printed sheet and compares the image and a document image to determine the quality of a printed image.
However, in comparison of the printed image and the document image for detection of a stain or a blur (waste detection), when an image of a stamp or the like is added to the document image, the part does not match, and the part is determined to be a stain. To prevent this, it is possible not to compare the stamp areas.
An example of addition of an image to a document image includes ground tint printing (for example, Japanese Patent Laid-Open No. 2008-186146). In the ground tint printing, a ground tint image including a combination of maintained dots and deleted dots is placed over the document image to generate an output matter. When the output matter is further copied, the small deleted dots become invisible, and the maintained dots are revealed. In this way, a copy can be determined.
For example, the ground tint is an image in which characters, such as “copying prohibited”, are revealed when copied (scanned). The ground tint is an image including an image of “copying prohibited” or the like and small dots for making the image inconspicuous, and the small dots are added to the entire surface of the sheet. In reading by a scanner, the small dots are not read (dots are that small), and “copying prohibited” that is inconspicuous in the original becomes conspicuous when copied (scanned) and can be viewed as if to be revealed. Conversely, an image of “copying prohibited” or the like can be formed by small dots, and other characters can be designed to be revealed. When the image is copied (scanned), “copying prohibited” or the like becomes white on a colored background.
However, when the printed image is detected to compare the detected image and the document image, not comparing the added area is not suitable because the ground tint is added to the entire surface (entire surface is not compared).
An additional image can be combined with the document image to compare the image and the printed image. However, small dots are not detected by scanning in the case of the ground tint, and it is determined that there is a blur in the printed image. Other than when the ground tint is added, these problems similarly occur when the document image includes an image not detected by scanning.
The present invention has been made in view of the circumstances, and an object of the present invention is to provide an image forming apparatus that can use information of a printed image and information of an image for printing to determine the printed image when the image for printing includes an image for which image detection is not possible.